Using a line of credit for loan consolidation

I've heard of strategies with people using a LARGE line of credit as their primary financing tool.  For example, 5 units at $200,000 each is their collateral for a $1,000,000 line.  When the line is paid down enough, they purchase their next property only using the line and avoiding traditional bank/lender financing.  I understand that the interest rate matters, but in general, what are your thoughts on this strategy?